RFR: 8310428: Add CollectedHeap::reserved_range() API to return reserved heap memory range

[Thomas Stüfe]

Hi Stefan,

thank you for your answer. Sorry, if I came over as frustrated. I was a
bit, but it is not ZGCs fault.

Please find remarks inline.

On Mon, Jun 26, 2023 at 8:37 AM Stefan Karlsson <stefan.karlsson at oracle.com>
wrote:

>
>
> On 2023-06-23 22:22, Thomas Stuefe wrote:
> > On Wed, 21 Jun 2023 15:30:17 GMT, Ashutosh Mehra <duke at openjdk.org>
> wrote:
> >
> >> Please review this patch to add a new API
> `CollectedHeap::reserved_range()`. The changes are extracted out from
> https://github.com/openjdk/jdk/pull/14520 as per the suggestion [here](
> https://github.com/openjdk/jdk/pull/14520#discussion_r1234991165).
> Support for ZGC is not implemented yet as it can have discontiguous heap
> regions and unlike other collectors, it does not set
> `CollectedHeap::_reserved`.
> >> I've had an off-line discussion with @fisk and created an investigative
> RFE based on his ideas. Please see #14520 and use that PR for further
> discussion on this topic.
> > Please let's have the discussion in the open.
> >
> > Nobody answered me, but I repeat, there is merit in knowing the
> contiguous region for collectors that have a contiguous reserved region.
>
> Sweden had a public holiday on Friday and people are heading out for
> vacation. We had to prioritize other discussions on Thursday.
>
> > I don't understand why we have to treat developers like children and
> force them to use really bad workarounds.
>
> What? There is a trade off between adding the wrong abstraction and
> cleaning up other parts of the code.
>

My original point was that the abstraction is correct for most GCs. An API
like the proposed one - with clear documentation, a clear "not supported"
return code the dev has to handle, and a companion capability query API -
would hopefully not be misused. It would also mimic what we do on a
platform level.

Thinking about this further, maybe we can find a better compromise, see
below.


>
> >
> > Just an example of the weird complexity this API omission causes. See
> class space reservation, where we try to reserve class space adjacent to
> the java heap:
> https://github.com/openjdk/jdk/blob/bfcca5eff96ac3cd72996b6c4865872c2da4de53/src/hotspot/share/memory/metaspace.cpp#L794-L796
> .
> >
> > 1) we don't know the address range of the heap
> > 2) so we have to misuse CompressedOops::encoding_range, which is a bad
> fit. Encoding range is not heap range. It can (often does) start before the
> heap, so we don't know the heap start address. It only tells us the heap
> range end, but that is an accident of implementation: semantically, it is
> incorrect since the encoding range end is not the end of the heap.
> > 3) Since we don't get the start address, we only know heap range end.
> That forces us to reserve the class space adjacent - following - the heap
> range.
> > 4) So, for zero-based heap reservation, we have to leave a (potentially
> very large - 1G) gap between zero-based encoding range end and the heap
> end:
> https://github.com/openjdk/jdk/blob/bfcca5eff96ac3cd72996b6c4865872c2da4de53/src/hotspot/share/memory/virtualspace.cpp#L548-L558
> This is bad, since it reduces the chance of getting zero-based heap. It
> also means we need to code intimate knowledge about class space (e.g. class
> space size) at heap reservation. Heap reservation should not have to know
> these things.
> > 5) if, instead, we had known the start address of the heap at (4), we
> could just reserve the class space in front to it. There would have to be
> no need for leaving a large gap between zero-based compressed oops encoding
> range end and heap end. Also, we would not need to have class-space
> knowledge at heap reservation.
> >
> > And all this complexity for a scenario that is not even supported on ZGC
> (CompressedOops) !
>
> I think this is an unfair assessment. The code grew to what it is today
> even when we had the CollectedHeap::reserved_range. I don't think we
> should blame ZGC that this hasn't been cleaned up.
>

Okay, that is true. But cleaning up this code would be easier with heap
cooperation.


>
> >
> > This is suboptimal. With a simple "CollectedHeap::reserved_range()" in
> combination with "CollectedHeap::is_contiguous()", this could be solved in
> about 10 lines of code.x§
> >
> > So,no, I don't think this discussion has come to a satisfying close.
>
> The door isn't closed for adding an API that helps clean up those parts.
> Could you add the required heap range to CompressedOops and used that
> for all the bullets above?


Thinking about this further, I'd like to remove consideration of
CompressedOops from klass range reservation completely. There is no
connection anymore. And this problem is not limited to klass reservation.
Reserving memory within a range of preferred addresses is done for other
cases too, nor is it limited to lower address ranges. Heap reservation
itself, for instance. Also, e.g.:
- in Shenandoah, certain helper structures are attempt-reserved in lower
ranges
- We could increase the chance of CDS runtime attaching the archive at the
preferred base address in the face of ASLR by splitting the reservation of
CDS archive (which has to be at address X) and the one of the follow-up
class space (which has to be just within klass encoding range).

I would like to have something like "os::reserve_in_range(from, to,
side-conditions)". Such an API would not need to know anything about the
heap but would benefit from knowing large reserved areas *like* the heap to
prevent unnecessary work. Typically the heap is the largest area at that
point in time. (Pity that we cannot use NMT for this).

I was experimenting with such an API. My idea was to use, on supported
platforms, OS information to find an address hole (procfs, VirtualQuery),
otherwise to fall back to ladder-probing the address space with mmap. The
latter can be sped up considerably by omitting the mmap calls for regions
we know are reserved.

A compromise could be adding an API like "is_in_reserved()" to
CollectedHeap. That would not be ideal, we'd still be looping, but the
expensive part - the mmaps - could be avoided. Hopefully, such an API could
be implemented in a fast way even for ZGC. Such an API would not assume a
contiguous heap.


> If not, and we really need to add this to
> CollectedHeap for some reason, then I think reserved_range need to be
> renamed to something that is bleeding obvious not going to be available
> for all GCs.
>
> StefanK
>
>
Thomas


> >
> > -------------
> >
> > PR Comment:
> https://git.openjdk.org/jdk/pull/14595#issuecomment-1604898333
>
>
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